1. Field of the Invention
The present invention generally relates to a method of melting a bump. In particular, the invention relates to a method of melting a bump that is improved to melt, in the dry process, a bump formed on a metal interconnection provided on a substrate or a bump formed an electrode of a semiconductor device such that the bump is sphered. The invention further relates to an apparatus for melting a bump improved to realize such a method of melting a bump. The invention still further relates to a method of manufacturing a semiconductor device including such a method of melting a bump.
2. Description of the Background Art
FIG. 12 is a plan view of a substrate on which solder bumps are produced. FIG. 13 is a partial cross sectional view along the line XIII--XIII in FIG. 12.
Referring to these figures, a substrate 2 is formed of resin such as glass epoxy or polyimide. A patterned copper foil 21 is mounted on one surface of substrate 2. A solder resist 22 is applied onto substrate 2 to cover copper foil 21. Laser drilling of the other surface of substrate 2 and laser drilling of solder resist 22 produce respectively an outer electrode portion 21b and an inner electrode portion 21a. Inner electrode portion 21a and outer electrode portion 21b are formed of plating of Ni/Au, Ni/Pd/Au or the like. Ball bonding of a solder wire to inner electrode portion 21a produces a solder bump 21c. Substrate 2 onto which flux is applied is passed through a reflow furnace to make solder bump 21c into a sphere. Substrate 2 and an electrode of a semiconductor chip are bonded together with solder bump 21c therebetween. The resultant product is entirely sealed with resin and an outer ball of solder is attached to outer electrode 21b. The product is then cut into separate packages as final products.
The conventional art discussed above is hereinafter described in detail with reference to figures.
FIG. 14 is a plan view illustrating a conventional art of applying flux for generating solder balls on a substrate. FIG. 15 is a plan view of a reflow furnace for melting solder. FIG. 16 is a cross sectional view along the line XVI--XVI in FIG. 15. FIG. 17 shows a temperature profile in the reflow furnace.
Referring to these figures, substrate 2 enters a melting reflow furnace 3 from a reflow furnace in-loader 31, and moves from the first zone to the fourth zone by use of a reflow carrier belt 33. Substrate 2 is heated by an upper heater 32a and a lower heater 32b. Solder bump 21c is melted to be sphered under the profile temperature conditions shown in FIG. 17, namely at 140-160.degree. C. for 70.+-.20 seconds in the second zone and at 200-220.degree. C. for 90 seconds or less in the fourth zone, and under a condition of an oxygen concentration of 1000 ppm or less. Substrate 2 is thereafter taken out from an unloader 34.
In order to remove the flux, a cleaning process with solvent and a drying process are carried out. A similar method is employed for a process of bonding solder to an outer electrode and a process of forming solder on elements on a printed circuit board.
The conventional art discussed above has the following problems.
The first problem is that four processes of applying the flux, reflowing, cleaning, and drying are required after generating the bump of the inner electrode portion, leading to reduction in productivity, and the residue of the flux deteriorates the reliability of the product.
The second problem is that contaminants in the cleaning fluid attach to the surface of the outer electrode on the bottom surface of the substrate in the cleaning process, and thus disturb sticking of solder paste used for bonding the outer ball after sealing process, resulting in peeling off of the ball.
The third problem is that the temperatures of bump 21c and substrate 2 rise to almost the same value within the reflow furnace so that the substrate is melted before the bump is melted if the reflow temperature is set at the glass transition point Tg or more. Therefore, medium temperature solder such as Sn--Ag and Sn--Sb and refractory metals such as Au and Cu cannot be used. Then, those products that satisfy both of the requirements of product quality and cost cannot be manufactured.